PROJECT SUMMARY Aging predisposes brain microglia to a pro-inflammatory state that is associated with cognitive and motor impairment, mood disorders, and disability. Furthermore, pro-inflammatory microglia in the senescent brain are hypersensitive to signals emerging from the peripheral immune system during infection. The result is an aberrant pro-inflammatory response in the aged brain that is intensified and longer-lasting. While the explanation for why microglia become pro-inflammatory and hypersensitive during aging is largely unknown, our preliminary data suggest intestinal dysbiosis and altered viscerosensory signaling may be involved. Therefore, the objective of the proposed research is to determine how an age-related change in intestinal short chain fatty acids (SCFAs) produced by the microbiota affects activity of the viscerosensory pathway and microglial cell activity. The central hypothesis is that SCFAs inhibit inflammatory microglia in the aged brain by normalizing communication in the gut-brain axis via Free Fatty Acid Receptor 2 (FFAR2) on intestinal epithelial cells. The SCFAs butyrate, acetate, and propionate provide energy, maintain integrity of the gut epithelium, and signal through FFAR2 and FFAR3 to regulate diverse physiologic systems, including inflammation and vagal afferent nerve activity. Our focus is on FFAR2 because it is highly expressed by intestinal epithelial cells and on leukocytes in the mesenteric lymph nodes. This anatomical distribution places FFAR2 at the interface separating the host from the gut microbiota and its bioactive products. Our approach is to manipulate intestinal SCFAs that are agonists for FFAR2 in aged mice and examine intestinal inflammation, viscerosensory signaling, and microglia activation. Furthermore, we will use transgenic mice to selectively knockout FFAR2 in intestinal epithelial cells and thereby determine its unique role in mediating communication in the gut-brain axis and microglia activation.